This invention relates to the field system of a magnet type D.C. motor such as an engine starting motor.
A prior-art field system of a magnet type D.C. motor has been as shown in FIG. 1. The figure illustrates one pole of the field system. Reference numeral 1 indicates the field assembly, namely, field system of the D.C. motor. The field system 1 comprises a cylindrical yoke 2 which is fabricated by press-working a soft steel plate, and a pole piece 3 which is made of a permanent magnet member such as a sintered ferrite compact that is bonded to the inner peripheral surface of the yoke 2 with a thermosetting resin, e.g., epoxy resin.
In operation, when an armature (not shown) arranged in the vicinity of the inner peripheral surfaces of such pole pieces 3 of the field system 1 is energized by a power supply (not shown), the armature is rotated by the magnetic field of the pole pieces 3. A reaction force due to the rotation develops in each pole piece 3, and it is transmitted to and received by the yoke 2.
The prior-art field system of the magnet type D.C. motor as described above has had the disadvantage that a magnetic flux produced at the energization of the armature interlinks with the field system 1, especially the yoke 2 of low magnetic reluctance, to exert an adverse influence on the magnetic characteristics of the field system 1. In addition, a high degree of bonding technique has been required for reliably bonding the pole piece 3 to a predetermined position of the yoke 2.